1. Field of the Invention
The present invention relates in general to a system and method for filtering a misfire-detection data-stream received from a multi-cylinder combustion engine, and more specifically to filtering a misfire data stream received during engine operation to yield an optimum measurement misfire-rate for accurate and rapid determination of the extent to which an engine is misfiring.
2. Related Art
Most automobiles employ a catalytic converter to reduce the amount of pollutants in engine exhaust. However, when a cylinder misfires (i.e., no or an incomplete combustion occurs) unburned fuel escapes into the exhaust system. This causes two problems: 1) tailpipe emissions of pollutants increase; and 2) high amounts of unburned fuel enter the catalytic converter causing excessive heat, which can permanently damage or destroy the catalytic converter.
The California Air Resources Board (CARB), as well as other state and federal agencies, have identified engine misfires as a contributor to vehicle emissions and a cause of poor catalyst performance. They have instituted regulations that require misfire detection systems be implemented in automobile on-board diagnostics to alert drivers (typically on the instrument panel), when a misfire measurement rate exceeds a threshold mandated by such agencies.
The aim of the regulations is to determine an accurate measurement of the misfire rate and to alert the driver as soon as possible when a misfire rate exceeds thresholds enumerated in the regulations. There are three important parameters to consider in order to carry out the regulations established by CARB and other agencies (hereinafter "CARB"). They are as follows: 1) threshold rate R.sub.th (set by CARB--the rate of misfire determined to cause emissions that are too high; 2) time constant (set by CARB--the length of a "window" over which measurements of the average misfire rate is taken); and 3) false alarm rate R.sub.fa, which represents a frequency of occurrence of normal firings which are incorrectly classified as misfirings. It should be noted that the false alarm rate R.sub.fa is calculated internally by misfire detection systems and should not be confused with false indications of failures, which occurs when a misfire detection system alerts an operator of a failure (that is, the misfire measurement rate exceeding the threshold) based on an incorrect determination that the misfire rate exceeds a predetermined threshold (such an incorrect determination, however, may be due to a high false alarm rate R.sub.fa).
CARB's desire to have the threshold rate (R.sub.th) as low as possible to minimize pollution is a basic competing interest with the automobile manufacturers' desire to have R.sub.th higher to avoid false indications of failure. Additionally, CARB prefers a short time constant (to obtain measurements quickly to avoid irreversible damage to the catalytic convert), whereas automobile manufactures prefer a longer time constant (to obtain accurate measurements). Long time constants provide measurements taken over a large number of samples and provide a more accurate estimation of the true misfire rate, but provide a slower response time when an unacceptable misfire rate occurs. Whereas short time constants provide a fast response time to unacceptable misfire rates, they often increase the likelihood of a false indication of failure. The reason is that to obtain a fast response a smaller measurement sample is used to evaluate misfire-rate. Temporary statistical fluctuations that occur during routine driving conditions are not filtered out of the true misfire-rate measurement and thus may appear to represent an unacceptable misfire rate. Whereas in-fact it is a false indication of failure.
Both the regulatory agencies and automobile manufacturers desire a low indication of failure. False indications are very costly to consumers and automobile manufactures in terms of unnecessary service costs (possible recall), wasted time, and so forth. Both regulatory agencies and automobile manufactures require a fast response time to misfire rates beyond threshold limits. Therefore, there is a need to measure misfire-rate accurately and rapidly to identify when engine misfire rates exceeds threshold limits.